How will they interface with the different APIs ? Or is it a single API defined by a standard ?

The first one means that most clients will play “whack a mole” with 20 APIs, trying to keep up with features. The second will be the lowest common denominator, limiting what can be sent between different clients. It would be the new “green bubble”.

Next, how do you identify people uniquely across different networks ? Phone number ? Email ? What happens if you’ve registered your Id in multiple places ? Or is it up to the sender to specify which network they wish to target ? Like someone@gmail.com@imessage ? The last one solves nothing. In case of multiple id registrations, should the network just keep trying round robin until it successfully delivers ? Or can I as a recipient register my preferred delivery network in case I never want Meta or Google to see my data ? Who maintains this central registry ? Will they do it for free ?

Now that we’ve established how to pass messages between networks, how do we secure them ? Do we use the iMessage model and use a central key repository ? Or do we implement a protocol (potentially per API) on how to acquire encryption keys ? Or do we simply skip encryption because security is hard ?

What about attachments ? Since most secure platforms use “per device” encryption, do we just send a 500GB attachment X times, one per device ? Do we limit the size of attachments ? iMessage solves this by encrypting it with a temporary key, and the attachment is then uploaded to Apples servers, and the temporary key is exchanged using normal messaging. Is that the way forward ? Will whoever handles it do it for free ? Do we trust them ?

What about Memoji/whatever the kids use ?

All of the above, and more, needs to be agreed on by all involved parties, which sets the lowest common denominator, either by a shared standard, or by reducing functionality for cross network messages. If it ends up complex enough to support all the features of modern instant messaging, it sets the bar rather high for new players. If it ends up simple, we have gained almost nothing over using SMS/MMS.

Things are never as simple as just exposing an API.

But lets say you are right, and all that the affected messaging services have to do is provide an API- will the regulators require them to document this API (and if so, what standards will the documentation have to follow?) Will they complain if the API changes too rapidly? Will the API have to support tall of the same features as the messaging service?

As they say, the devils in the details, and right now I'm not seeing any details.